GB2193843A

GB2193843A - Medium voltage switches

Info

Publication number: GB2193843A
Application number: GB08717598A
Authority: GB
Inventors: Geoffrey Auton
Original assignee: ABIMER
Current assignee: ABIMER
Priority date: 1986-07-25
Filing date: 1987-07-24
Publication date: 1988-02-17
Anticipated expiration: 2007-07-24
Also published as: GB8717598D0; GB2193843B

Abstract

A single, earthed, axially immovable shaft 12 carries at least one switching blade 10 fast thereon. The switching blade has two identical arms which are spaced angularly apart from one another, the arms extending radially from the shaft. For each switching blade on the shaft, there are at least three terminal contacts 14, 16, 18 spaced angularly apart from one another and lying on a circle 22 centred on the shaft axis. Rotation of the shaft results in a simultaneous double make or double break between the arms and the relevant terminal contacts. A ring main unit having a number of such switching arrangements is also described, with three parallel shafts each bearing three blades and positioned in a SF6- filled housing. <IMAGE>

Description

SPECIFICATIOM Medium voltage switches BACKGROUND TO THE INVENTION THIS invention relates to medium voltage switching arrangements and to ring main units in which such switching arrangements are used.

By "medium voltage" is meant a voltage in the range lkV to 36kV.

Conventionally, a ring main unit has three switches, one for each phase and each of which has an ON, an OFF and an EARTH position. The known switches used in such units are generally one of two types. The first type has a single break contact blade or blades which pivot between their respective positions, while the other type has a sliding contact. The known ring main units incorporating such switches are usually large in size, and it is believed that the switching action of the switches in the units is less than optimum.

SUMMARY OF THE INVENTION A first aspect of the invention provides a medium voltage switching arrangement which comprises a single, axially immovable, earthed shaft, at least one electrically conductive switching blade mounted fast on the shaft, the switching blade having two identical arms spaced angularly apart from one another and extending radially from the shaft, and, for each switching blade, at least three terminal contacts spaced equiangularly from one another on an imaginary circle centred on the shaft axis, the terminal contacts being equidistant from the shaft axis, wherein the shaft is rotatable about its axis to move the or each switching blade between a first, contact-making position in which the arms of the switching blade simultaneously make bridging contact between any chosen two of the terminal contacts and a second, contact breaking position in which the arms of the switching blade simultaneously break contact with the terminal contacts.

In one embodiment, each contact blade is boomerang shaped with the shaft passing through its centre and with its arms including an angle of 1200 between them, there being three terminal contacts each spaced from the other two by an angle of 1200 for each switching blade. There may be three switching blades spaced apart from one another on the shaft.

A second aspect of the invention provides a ring main unit which incorporates a plurality of such switching arrangements.

A third aspect of the invention provides a ring main unit which comprises a sealed cabinet, three spaced apart, rotatable shafts extending through the cabinet each shaft carrying a plurality of switching blades fast thereon, each switching blade having aligned with it three terminal contacts which are mounted fixedly with respect to the cabinet and which are spaced angularly apart on a circle which is centred on the axis of the shaft carrying the switching blade in question, the blade being shaped to make and break bridging electrical contact between any chosen two of the associated terminal contacts on appropriate rotation of the shaft.

There may be three switching blades on each shaft, and each switching blade may be boomerang shaped with its shaft passing through its centre, the blade having two identical arms spaced apart from one another by 1200.

Preferably, there are three terminal contacts associated with each switching blade such terminal contacts being spaced apart from one another by 1200.

For compactness of the ring main unit, it is preferred that the three shafts are disposed at the corners of an imaginary, equilateral triangle. Each shaft may carry first, second and third switching blades, the first switching blades of the three shafts being aligned with one another in the axial direction and the second and third switching blades being likewise aligned. One terminal contact may serve all three first switching blades, one terminal contact likewise serving all three second switching blades and one terminal likewise serving all three third switching blades.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates, in schematic and simplified form, a switching arrangement according to the invention; Figure 2 shows a perspective external view of a ring main unit according to the invention; Figure 3 shows a schematic internal view of the ring main unit; Figure 4 shows a cross section at line 4-4 in Figure 3; and Figure 5 shows how contact is made between a switching blade and a terminal contact.

SPECIFIC DESCRIPTION Referring firstly to Figure 1, there is a switching blade 10 which is boomerang shaped and which is mounted fast on an earthed shaft 12 passing centrally through the boomerang shape. Three terminal contacts 14,16 and 18 are located on an imaginary circle 22 which is centred on the axis of the shaft 12. The terminal contact 14 is the "ON" terminal, the terminal contact 16 is the "LIVE" terminal and the terminal contact 18 is the "EARTH" terminal.

Clearly, with the boomerang shaped blade 10, it is possible to rotate the shaft 12 to appropriate positions in which bridging electrical contact is made between any chosen two of the terminal contacts 14 to 18. As mentioned previously, one of the major advan tages of this switching arrangement is the fact that when the shaft is rotated out of a contact-making position (as illustrated in broken outline) to a contact-breaking position (as illustrated in full outline), contact breaking in fact takes place at two different locations simultaneously. The result is an increase in the effective arc path, and, it is believed, less chance of an arc-related failure.

In the open or disconnected position when the blade is separated from the terminal contacts 14 and 16 there is established an 'isolating distance' between the LIVE and the ON contact, the integrity of which distance is of great importance for the operational safety of the switch. By this is meant the freedom from the possibility of electrical flash-over from the LIVE to the ON contact, and the possibility of this is greatly reduced by the fact that the shaft 12 is effectively earthed. This has the effect of introducing an earthing component, shaft 12, in the electrical field which exists between the ON terminal contact 14 and the LIVE terminal contact 16 which modifies the stress pattern in the electrical field in such a way as to increase the electrical flash-over characteristic of the isolating distance.

Figures 2 to 5 illustrate a ring main unit incorporating switching arrangements employing the above principles.

In Figure 2, the ring main unit is shown to comprise a housing 24 mounted on legs 26.

The ends of three shafts 28 protrude through the front wall 30 of the housing. In use, operating handles (not shown) are secured to the protruding shaft ends to facilitate rotation thereof. Also the front wall 30 can have appropriate instructions and mimic diagrams applied to it to assist the operator. The unit includes a lid 32 which can be locked in position over the front wall 30 to prevent unauthorised tampering with the operating handles and to weatherproof the housing.

Inside the housing 24 is a cabinet 34 through which the shafts 28 extend, parallel to one another and in rotatable fashion. The shafts are supported in appropriate sealed bearings in frame members (not illustrated) which span across the front of the cabinet and in the rear wall 42 of the cabinet.

As illustrated by Figures 3 and 4, each shaft 28 carries three boomerang shaped switching blades 10 which are arranged along the length of the shaft in spaced reiationship to one another. The shaft is insulated with an epoxy or other insulating coating 43 and includes insulating, arc-obstructing projections 44 between the blades 10. There are three terminal contacts associated with each blade on each shaft.Referring specifically to one of the blades 10 on a shaft 28A (Figure 4), the first of these contacts 46 extends through the wall of the cabinet below the shaft 28A, the second earth contact 48 extends through the wall of the cabinet at an angular spacing of 120O from the first and the third contact 58 is fixed to a frame member which extends lengthwise through the cabinet between the front frame members previously referred to and the rear wall 42. Similar terminal contact arrangements are present for each of the other blades on each shaft 28, with the spacing of the terminal contacts corresponding exactly to that of the associated blades and with suitable busses connecting corresponding terminal contacts.

It will be noted that the terminal contact 58 is common to all three blades at the same longitudinal position along each of the shafts 28.

As stated previously, the terminal contacts 46 and 48 extend through the wall of the cabinet 34. Outside the cabinet, suitable connections are provided for the connection of the appropriate electrical cables to the terminal contacts. Although the earth contacts 48 are described as passing through the wall of the cabinet, in some embodiments they can extend to the wall without passing through.

The interior of the cabinet is sealed and is filled with SF6 gas (sulphur hexafluoride) which is conventional in heavy duty switching units of the kind here under consideration. Other suitable electrically insulating liquids or gases can be used instead of SF6. Where the.terminal contacts pass through the wall of'the cabinet, appropriate sealing arrangements are provided.

Each blade operates in the manner described above with reference to Figure 1 of the drawings i.e. appropriate rotation of the relevant shaft 28 brings the blade 10 in question into a position of bridging contact between two chosen terminal contacts to complete the appropriate electrical circuits or to establish an earth connection. As described previously, contact breaking takes place at two locations simultaneously with advantageous arcing characteristics.

Figure 5 illustrates schematically how each contact is made. The blades 10 are made of copper and each end of the blade is fitted with a contact element 60 connected electrically to the blade. Each terminal contact is fitted with two spaced contact elements 62 and 64 which are biased towards one another by means of Belleville springs 66. The ends of the contact elements 62 and 64 are formed with inwardly directed protrusions 68. When the relevant shaft 28 is rotated to a position of contact, the contact element at the end of the blade 10 slides into the space between the contact elements 62 and 64 and good electrical contact is established by the spring bias of the elements 62 and 64 and the protrusions 68.

An important advantage of a ring main unit which incorporates the rotary switching arrangement of the invention is the fact that it can be made relatively compact when com pared to conventional ring main units having conventional switching arrangements.

The compactness of the unit can result in reduced weight and easier siting and transportation.

Although not illustrated, provision can be made for appropriate fuses which will be accommodated in the cabinet 34 or in a separate chamber forming part of the unit and means may be provided to open one or more of the switches automatically in the event of a fuse blowing.

Claims

1. A medium voltage switching arrangement which comprises a single, axially immovable, earthed shaft, at least one electrically conductive switching blade mounted fast on the shaft, the switching blade having two identical arms spaced angularly apart from one another and extending radially from the shaft, and, for each switching blade, at least three terminal contacts spaced equiangularly from one another on an imaginary circle centred on the shaft axis, the terminal contacts being equidistant from the shaft axis, wherein the shaft is rotatable about its axis to move the or each switching blade between a first, contact-making position in which the arms of the switching blade simultaneously make bridging contact between any chosen two of the terminal contacts and a second, contact breaking position in which the arms of the switching blade simultaneously break contact with the terminal contacts.

2. The medium voltage switching arrangement of claim 1 in which each contact blade is boomerang shaped with the shaft passing through its centre and with its arms including an angle of 1200 between them, and in which there are three terminal contacts each spaced from the other two by an angle of 1200 for each switching blade.

3. The medium voltage switching arrangement of either one of the preceding claims in which there are three switching blades spaced apart from one another on the shaft.

4. The medium voltage switching arrangement of any one of the preceding claims in which the shaft is insulated.

5. A ring main unit comprising a plurality of switching arrangements according to any one of the preceding claims.

6. A ring main unit which comprises a sealed cabinet, three spaced apart, rotatable shafts extending through the cabinet each shaft carrying a plurality of switching blades fast thereon, each switching blade having aligned with it three terminal contacts which are mounted fixedly with respect to the cabinet and which are spaced angularly apart on a circle which is centred on the axis of the shaft carrying the switching blade in question, the blade being shaped to make and break bridging electrical contact between any chosen two of the associated terminal contacts on appropriate rotation of the shaft.

7. The ring main unit of claim 6 in which the shaft is axially immovable and earthed.

8. The ring main unit of either one of claims 6 or 7 in which each shaft carries three switching blades.

9. The ring main unit of any one of claims 6 to 8 in which each switching blade is boomerang shaped with its shaft passing through its centre, the blade having two identical arms spaced apart from one another by 1200.

10. The ring main unit of claim 9 in which there are three terminal contacts associated with each switching blade such terminal contacts being spaced apart from one another by 1200.

11. The ring main unit of claim 10 in which the three shafts are disposed at the corners of an imaginary, equilateral triangle.

12. The ring main unit of claim 11 in which each shaft carries first, second and third switching blades, the first switching blades of the three shafts being aligned with one another in the axial direction and the second and third switching blades being likewise aligned.

13. The ring main unit of claim 12 in which one terminal contact serves all three first switching blades, one terminal contact likewise serving all three second switching blades and one terminal likewise serving all three third switching blades.

14. A switching arrangement substantially as herein described with reference to the accompanying drawings.

15. A ring main unit substantially as herein described with reference to the accompanying drawings.